Adders (a.k.a. Grass Snakes)

Adders are different from grass snakes:

The adder (a.k.a. viper), vipera berus, is a venomous snake found throughout Great Britain. They are not especially dangerous as they normally only bite when disturbed. The bites are painful but rarely fatal;

The non-venomous grass snake, Natrix natrix, is found mainly in England and Wales.

According to Wiki, smooth snakes are also found in southern England as well as some isolated colonies of Aesculapian snakes, around Regent's Park in London and the Welsh Mountain Zoo, near Conwy, which have escaped from captivity. Both of these are non-venomous.

DC